1. Field of the Invention
This invention relates to an improvement of an internal combustion engine having a cylinder block formed of aluminum alloy, and more particularly to a crankshaft bearing structure arranged to reduce noise due to bearing metals located in contact with the outer surface of a crankshaft.
2. Description of the Prior Art
In connection with automotive internal combustion engines, it is a recent tendency to employ a cylinder block formed of aluminum alloy and produced by die casting. Experiments have revealed that, in such engines, bearing metals located in contact with crankshaft main journals considerably contribute to an increase in engine noise, particularly of a high frequency range of from 10.sup.3 to 10.sup.4 Hz. This noise increase is believed to be caused by the fact that the existence of the bearing metals does not provide the right cylindrical shape of a bearing surface which is in direct contact with the crankshaft main journals, while allowing the bearing metals themselves to generate noise.